In device fabrication, insulating, semiconducting, and conducting layers are formed on a substrate. The layers are patterned to create features and spaces. The minimum dimension or feature size (F) of the features and spaces depends on the resolution capability of the lithographic systems and materials used. The features and spaces are patterned so as to form devices, such as transistors, capacitors, and resistors. These devices are then interconnected to achieve a desired electrical function, creating an integrated circuit (IC) or chip.
Fuses are employed to alter interconnections in an IC after standard processing is completed. The ability to alter interconnections gives IC manufacturers flexibility to customize a standard IC design in order to accommodate a customer's specific needs. Fuses have also been used to provide connections to redundancy circuits, replacing defective circuits to improve yield. For example, fuses are used to replace defective word- and/or bit-lines with redundant lines (redundancy activation) in a random access memory (RAM) IC.
One type of fuse, referred to as laser blowable fuses, is typically formed at or near the surface of the IC. A laser beam striking the fuse material renders the fuse portion non-conductive, thereby inhibiting current from flowing through. Laser blowable fuses are widely used because they are relatively simple to fabricate.
The fuses are generally formed on the surface of the IC. The IC and the fuses are covered by a hard and soft passivation layer to protect the devices from the ambient. A barrier layer is provided to isolate the fuse from the soft passivation layer. The soft passivation layer comprises, for example, photosensitive polyimide (PSPI). To access a fuse, an opening is created in the PSPI. The opening is referred to as the terminal via (TV) opening.
Formation of the TV opening is achieved by lithographic techniques. Such techniques include selectively exposing the PSPI with an exposure source and mask. The mask contains a pattern corresponding to TV openings. The PSPI is then cured to render the PSPI thermally stable. After curing, a reactive ion etch (RIE) is performed. The RIE removes the hard passivation and other insulating layers in exposed regions, creating the TV openings to the fuses.
The lithographic resolution of the PSPI, which determines the minimum feature size of the TV, depends on the photosensitive polymers that are currently available. Currently available photosensitive polymers are able to reliably define small TV openings of about 10 um.
In future IC designs, it is important to provide smaller TV openings to enable further miniaturization in chip size. However, currently available PSPI are unable to accomodate advance fuse designs that occupy less area.
As evidenced from the foregoing discussion, it is desirable to enable smaller TV windows to accommodate advance fuse designs which occupy less space.